Static random access memory (SRAM) is a common memory device that does not require constant refresh to retain stored data. An example SRAM has multiple cells that are made of six transistors, such as metal oxide semiconductor (MOS) transistors. The example cell has two cross-coupled inverters that form a latch circuit. The latch is coupled between high and low voltage rails. The inverters are coupled to corresponding access transistors. The latch is connected between a high storage node and a low storage node. Each of the storage nodes is coupled to a bit line or a complementary bit line via an access transistor. The gate terminals of the access transistors are connected to a word line. The storage nodes are insulated when the access terminals are deactivated by applying a low voltage to the wordline thus allowing the cell to hold the data without refresh.
In order to write data into the cell, the access terminals are coupled to a write driver which discharges the bit line. A high voltage is applied to the wordline of the desired cell to be written. The write occurs when a low voltage is applied to the access terminal of one of the access transistors. As a result of discharging the bit line, the latch flips to a different state and holds the opposite high or low storage node.
As integrated circuits have increased in gate density and complexity, lower power requirements for the circuits are desirable. As a result, designers have sought to lower the high voltage required on integrated circuit designs. The lowering of the voltage requirements in circuit designs has made it more difficult to operate SRAM circuits due to the need to ensure a stable cell while at the same time allowing the cell to be writeable. In order to ensure operation of SRAM circuits using lower operating voltages, designers employ a write assist circuit. A write assist circuit uses a capacitor to pull the voltage on the access terminals below the low voltage to a level which is lower than the point at which the cell changes state. This level can be negative, but is not required to be so. By giving a larger range to the discharge level, the circuit thus allows an SRAM cell to be developed with good stability while retaining a robust ability to be written.
However, due to the wide range of operating conditions of SRAM circuits such as usage, voltages, temperatures, etc., write assist circuits produce a wide range of voltages. The range of voltages can result in adverse performance. For example, if the voltage is too negative, latch up may occur and other unintended bit lines may be discharged (bit cell disturbs). On the other hand, if the voltage is too close to the trip point of the cell, the bit cell will not write over all operating conditions. The problems of the wide range of voltages provided by write assist circuits are increased with the use of circuits such as the SmartReflex device by Texas Instruments, which adapts the voltage of a circuit based on operating conditions. The wide range of voltages results in difficulties in controlling the voltages output by write assist circuits.